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/* Based on Frosbite Unified Volumetric.
* https://www.ea.com/frostbite/news/physically-based-unified-volumetric-rendering-in-frostbite */
/* Step 3 : Integrate for each froxel the final amount of light
* scattered back to the viewer and the amout of transmittance. */
uniform sampler3D volumeScattering; /* Result of the scatter step */
uniform sampler3D volumeExtinction;
flat in int slice;
layout(location = 0) out vec4 finalScattering;
layout(location = 1) out vec4 finalTransmittance;
void main()
{
/* Start with full transmittance and no scattered light. */
finalScattering = vec4(0.0);
finalTransmittance = vec4(1.0);
vec3 tex_size = vec3(textureSize(volumeScattering, 0).xyz);
/* Compute view ray. */
vec2 uvs = gl_FragCoord.xy / tex_size.xy;
vec3 ndc_cell = volume_to_ndc(vec3(uvs, 1e-5));
vec3 view_cell = get_view_space_from_depth(ndc_cell.xy, ndc_cell.z);
/* Ortho */
float prev_ray_len = view_cell.z;
float orig_ray_len = 1.0;
/* Persp */
if (ProjectionMatrix[3][3] == 0.0) {
prev_ray_len = length(view_cell);
orig_ray_len = prev_ray_len / view_cell.z;
}
/* Without compute shader and arbitrary write we need to
* accumulate from the beginning of the ray for each cell. */
float integration_end = float(slice);
for (int i = 0; i < slice; ++i) {
ivec3 volume_cell = ivec3(gl_FragCoord.xy, i);
vec4 Lscat = texelFetch(volumeScattering, volume_cell, 0);
vec4 s_extinction = texelFetch(volumeExtinction, volume_cell, 0);
float cell_depth = volume_z_to_view_z((float(i) + 1.0) / tex_size.z);
float ray_len = orig_ray_len * cell_depth;
/* Evaluate Scattering */
float s_len = abs(ray_len - prev_ray_len);
prev_ray_len = ray_len;
vec4 Tr = exp(-s_extinction * s_len);
/* integrate along the current step segment */
Lscat = (Lscat - Lscat * Tr) / s_extinction;
/* accumulate and also take into account the transmittance from previous steps */
finalScattering += finalTransmittance * Lscat;
finalTransmittance *= Tr;
}
}
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